Confocal μ-XRF and μ-XAFS studies of fractured granite following a radiotracer migration experiment

We perform spatially resolved X-ray absorption fine structure and fluorescence investigations with a micrometer-scale resolution (μ-XAFS, μ-XRF) for neptunium speciation in a fractured granite bore core stemming from the Swedish Aspo Hard Rock Laboratory following a radiotracer experiment. These investigations are relevant for safety assessment of nuclear waste disposal in assessing mechanisms leading to immobilization of actinide elements in the environment. In order to probe micro-volumes below the sample surface, a confocal irradiation-detection geometry is employed. Upon scanning sample areas at defined depths below the sample surface, cross sections visualizing the distribution of elements are recorded. By varying the energy of the beam at a constant sample position, Np L3 edge XANES spectra of a selected sample volume are recorded. Results reveal that Np, originally introduced as Np(V) in the tracer cocktail, is reduced to Np(IV). The tracer Np is observed to be located near or on fissures in the granite and associated with Zn. Further work is needed to support the conclusion that the distribution of Np results from local geometries, surface areas, and flow paths in the granite. (authors)